It is known to make cellular or foamed plastic masses by incorporating a volatile organic liquid, which volatilizes under the action of heat to form a gas, with a thermoplastic material and thereafter heating the material at elevated temperatures whereby the vapor of the volatile liquid expands the thermoplastic material to form a cellular mass, i.e., a foamed sheet. For example, expandable styrene polymer particles containing 1- to 20-weight percent of a volatilizable expanding agent such as propane, butane, pentane, methylchloride or a variety of chlorofluorocarbon compounds are known. These expandable styrene polymer particles produce/pre-expanded, cellular polymers or foams by heating the material at a temperature above the softening point of the polymer. Such techniques include the mixture of the polymer granules with the blowing agent, and the subsequent extrusion of the heat plasticised mixture to form a sheet which is then employed for packaging processes either as a flat sheet or formed by a heat fabrication technique to provide a desired shaped foamed article.
A variety of volatile blowing agents have been employed in the preparation of such foams. For example, lower liquid alkyl hydrocarbons have been employed in the blowing of polystyrene foams, but such hydrocarbons are flammable and for this reason elaborate and expensive precautions must be taken when it is used. In addition, when a significant amount of hydrocarbon residue remains in the product, the product's usefulness in certain applications is limited.
Thus, the blowing agents used in the production of such foams have usually been saturated chlorofluorocarbon (CFC) compounds such as dichlorodifluoromethane. However, saturated chlorofluorocarbons are suspected of destroying the earth's protective ozone layer by migrating through the troposphere to the stratosphere and catalyzing ozone-destroying chain reactions. Thus, it has been a desideratum to obtain a volatile blowing agent compound or mixture which is not damaging in the atmosphere, yet provides a useful blowing effect.
As mentioned above, hydrocarbon vapors have significant problems with respect to flammability, which render the use of any significant amount of such vapors inordinately expensive.. While unsaturated chlorofluorocarbons are much less harmful to the ozone layer, such compounds will not always blow properly so as to produce a usable product. While mixtures of hydrocarbons and chlorofluorocarbons have been proposed, a safe, effective blowing agent has yet to be produced, prior to this invention.